This invention relates to semiconductor device manufacture and in particular to the construction of devices in which one or more epitaxially grown layers are required to be present in an in-filled well formed in one surface of a semiconductive body, but not on the surface surrounding that well, or of the complementary structure in which the layers are required to surround a mesa but not to be present on top of it. Such structures are desirable for instance in the use of certain forms of circuit device that incorporate both all-electronic and electro-optic elements on a common substrate chip, such as optical receiver chips that include a photodetector and a front-end amplifier. In the manufacture of such devices it is generally advantageous if the growth can be arranged so as to produce a substantially planar surface at the end of the growth of semiconductor material. For one reason, this enables the use of high quality (contact) photolithography.
One approach to providing structures of this kind is described in United Kingdom Patent Specification No. GB 2 222 720, to which attention is directed. That specification describes a method of manufacture in which a well is etched prior to the growth of first and second sets of epitaxial layers, the first set in which FET's may subsequently be constructed and the second set for the construction of a photodiode. Then the well area is masked while the second set of layers is removed from the area surrounding the well. By making the depth of the well approximately equal to the aggregate thickness of the second set of layers, the height of material surrounding the well is reduced to a level substantially flush with the top of the material under the mask in the well. There is however a problem of registration between the mask and the well. Clearly a substantially flush top surface would not result if the mask were allowed to overlap the side wall of the well at any point in its perimeter, and so the mask is made deliberately smaller than the well in order to leave an adequate safety-margin for alignment errors. This safety-margin results in the formation of a trench between the side walls of the well and the masked area within the trench. For some applications the presence of such a trench presents no particular problem, but for others its presence would be a distinct drawback.